Thread forming fasteners for ultrasonic load measurement and control

ABSTRACT

An ultrasonic load measurement transducer ( 14 ) is mated with a thread-forming fastener ( 12 ) to provide a load indicating thread-forming fastener ( 10 ) that can be used for precise and reliable assembly of critical bolted joints, such as those in the automobile and aerospace industries, among others. Steps can then be taken to accurately measure and control the load in the thread-forming fastener ( 10 ) during tightening, and to inspect the load in the thread-forming fastener ( 10 ) after assembly. A similar result can be achieved for a thread-locking fastener by mating an ultrasonic transducer with the thread-locking fastener assembly.

BACKGROUND OF THE INVENTION

This invention relates to load indicating fasteners that are“thread-forming” (also referred to as “thread-rolling” or “self-tapping”fasteners), methods for making load indicating thread-forming fasteners,and methods for measuring the load in thread-forming fasteners.

Thread-forming fasteners are well known in many industries, such as inhigh-volume automotive assembly. Examples of such fasteners aredescribed in U.S. Pat. No. 5,242,253 (Fulmer), issued Sep. 7, 1993, forexample. Such fasteners are also marketed commercially, for example, byReminc, Research Engineering and Manufacturing Inc., Middletown, R.I.,USA, under the trademark “Taptite” and “Taptite 2000”, and a descriptionof such fasteners can be found in their product literature, entitled“Taptite 2000 Thread Rolling Fasteners”.

The major advantage of thread-forming fasteners is that they can beinstalled directly into a drilled hole, eliminating the cost of tappingthe hole. Additionally, the thread formed by a thread-forming fastenerhas very tight tolerance since it is formed by the fastener itself andtherefore forms a better nut.

Although thread-forming fasteners have been used in numerousapplications in the automotive and aerospace industries to reduce cost,such fasteners are generally restricted to non-critical or less-criticalapplications. The difficulty in controlling the tightening processprevents their use in critical applications.

The primary reason for this is that the thread-forming process requirestorque, in addition to the tightening torque, and this thread-formingtorque varies significantly with hole tolerance, material, frictionconditions, etc. As a result, the precise tightening of a thread-formingfastener to a specified torque into a blind hole, where the thread isstill being formed as the bolt is being tightened, will result in a 3sigma load scatter of typically +/−50%, which is unacceptable incritical applications.

SUMMARY OF THE INVENTION

For some time, ultrasonics has been used to accurately measure the loadin bolts. Initially, removable ultrasonic devices were the most commonlyused. More recently, low-cost permanent ultrasonic transducers, whichcan be permanently attached to one end of the fastener, have come to beused. Permanent fasteners of this type are described, for example, inU.S. Pat. No. 4,846,001 (Kibblewhite), issued Jul. 11, 1989, U.S. Pat.No. 5,131,276 (Kibblewhite), issued Jul. 21, 1992, U.S. ProvisionalPatent Application No. 60/264,877 (Kibblewhite), filed Jan. 29, 2001,and International Application No. PCT/US02/03920 (Kibblewhite), filedMay 17, 2002, the subject matter of which is incorporated by referenceherein.

In accordance with the present invention, it has been determined thatsuch ultrasonics can be mated with an otherwise conventionalthread-forming fastener to provide a load indicating thread-formingfastener that can be used for precise and reliable assembly of criticalbolted joints, such as those in automobile engines (e.g., cylinderheads, connecting rods, main bearings, etc.), drive trains, steering,brakes, suspensions, and a variety of other applications, includingaerospace applications.

Steps can then be taken, using equipment and methods that are otherwiseknown and conventional, to accurately measure and control the load inthe thread-forming fastener during tightening, and to inspect the loadin the thread-forming fastener after assembly.

For further detail regarding preferred embodiments for implementing theimprovements of the present invention, reference is made to thedescription which is provided below, together with the followingillustrations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a typical load indicating thread-formingfastener which is produced in accordance with the present invention.

FIGS. 2 and 3 are graphs showing typical load and torque characteristicsplotted against the angle of rotation of the load indicatingthread-forming fastener of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a typical embodiment of a load indicating thread-formingfastener which is produced in accordance with the present invention. Inthis illustrative example, the load indicating thread-forming fastenerhas been implemented in conjunction with an otherwise conventional“Taptite” fastener, which is commercially available from Reminc,Research Engineering and Manufacturing Inc., Middletown, R.I., USA. Itis to be understood, however, that this embodiment is shown only forpurposes of illustration, and that the load indicating thread-formingfastener of the present invention can also be implemented using any of avariety of known and available load indicating devices, coupled orcombined with any of a variety of known and available thread-formingfasteners.

In the illustrative embodiment of FIG. 1, the load indicatingthread-forming fastener 10 generally includes a fastener 12 (e.g., theabove-mentioned “Taptite” fastener) and a permanent piezoelectricpolymer film transducer 14 (e.g., of the type disclosed in theabove-mentioned U.S. Pat. No. 4,864,001, issued to Kibblewhite) attachedto one end. The fastener 12 includes a head 16, which can be suitablyengaged by a tool (not shown) for applying torque to the fastener 12,and a thread-forming body portion 18. As disclosed in U.S. ProvisionalPatent Application No. 60/264,877 (Kibblewhite) and InternationalApplication No. PCT/US02/03920 (Kibblewhite), the transducer 14 canfurther include a two-dimensional high-density bar code (not shown)applied to the top electrode 20 of the transducer 14, for purposes offacilitating the subsequent steps taken to obtain an indication oftensile load, stress, elongation or other characteristic of the fastener12 during a tightening operation, or at various other times during theservice life of the fastener 12, as will be discussed more fully below.

As an example, the transducer 14 can be implemented using a thinpiezoelectric polymer sensor (e.g., a 9 micron thick, polyvinylidenefluoride copolymer film, of the type manufactured by MeasurementSpecialties Inc., Valley Forge, Pa.) permanently, mechanically andacoustically attached to an end surface 22 of the fastener 12. The topelectrode of the transducer 14 can be implemented as a thin metallicfoil (e.g., an approximately 50 micron thick, type 316, full-hard, dullor matte finished stainless steel) which has been treated to provide ablack oxide finish, which is then preferably provided with a black oxidetreatment to provide an extremely thin, durable, corrosion resistant andelectrically conductive, black coating. A high-resolution bar code canthen be marked on this surface by removing selected areas of the coating(e.g., by conventional laser ablation techniques) to provide a highcontrast mark easily read with conventional, commercially availableoptical readers.

It is again to be understood that such implementations are describedonly for purposes of illustration, and that any of a variety oftransducer configurations can be used to implement the transducer 14applied to the fastener 12, as desired. For example, the ultrasonictransducer 14 can be implemented as an oriented piezoelectric thin film,vapor deposited directly on the head of the fastener 12, as is describedin U.S. Pat. No. 5,131,276 (Kibblewhite), issued Jul. 21, 1992. As afurther alternative, the ultrasonic transducer 14 can be implemented asa piezoelectric polymer film, chemically grafted on the head of thefastener 12, as is described in U.S. Provisional Patent Application No.60/264,877 (Kibblewhite), filed Jan. 29, 2001, and InternationalApplication No. PCT/US02/03920 (Kibblewhite), filed May 17, 2002. Itwill be readily understood that other alternative implementations arealso possible.

In the embodiment illustrated in FIG. 1, the ultrasonic transducer 14 ispermanently attached to the head 16 of the fastener 12, as described inthe above-referenced patents issued to Kibblewhite. An essentially flat,or spherically radiused surface 24 is provided on at least a portion ofthe threaded end of the fastener to provide an acoustically reflectivesurface to reflect the ultrasonic wave transmitted by the transducerback to the transducer. Load is then measured using standard, pulse-echoultrasonic techniques, which are themselves known in the art anddescribed, for example, in the above-referenced patents issued toKibblewhite. Load control accuracies of +/−3% have been achieved whentightening thread-forming fasteners into blind holes during both thefirst and subsequent tightenings.

In an alternative embodiment, an essentially flat surface is provided onthe head 16 of the thread-forming fastener 12 and a removable ultrasonictransducer is temporarily attached to the fastener for the purpose ofmaking load measurements. The threaded end of the fastener 12 isidentical to the previous embodiment with the permanent ultrasonictransducer.

In practice, heat is generated as a result of the thread-forming workthat takes place during the thread-forming run-down stage of theinstallation of a thread-forming fastener. This results in a slightincrease in temperature in both the fastener (the bolt) and theresulting joint. This increase in temperature can cause errors in theultrasonic load measurements to be taken because of thermal expansioneffects. For this reason, when using ultrasonics for inspecting the loadin a fastener, it is usual to measure the temperature of the fastener orthe joint in order to compensate for the effects of thermal expansion.

However, in conjunction with a thread-forming fastener, the averagetemperature increase due to the heat generated during thread-formationcan not be measured directly during the installation process and issubject to variations in material, friction, and heat conductionproperties of the joint components. Without compensation, this thermaleffect can result in inaccuracies of load measurement on the order of 5%to 20%, depending on the bolt, the joint and the assembly process beingused.

FIGS. 2 and 3 show typical load and torque characteristics plottedagainst the angle of rotation of a typical bolt. FIG. 2 shows thetightening curves for a typical through-hole application, in which thetorque reduces after the thread is formed through the entire hole. FIG.3 shows the tightening curves for a typical blind hole application, inwhich the thread is still being formed as the bolt is tightened.

Further in accordance with the present invention, more accurate loadmeasurements in the thread-forming load indicating fasteners areprovided by eliminating the effects of fastener heating resulting fromthe thread-forming process. This is achieved by measuring the load (oracoustic time-of-flight) value immediately prior to the load-inducingstage of the assembly process, and by using this measured value as thezero-load reading.

The load-inducing stage of the assembly process can be detected by anyone of a variety of methods. For example, an increase in load above apredetermined threshold, a change in the increase in load with time,angle of rotation of the fastener or torque, an increase in torque abovea predetermined threshold, or a change in the increase in torque withtime, angle or load can be detected. Irrespective of the method used todetect the load-inducing stage of the assembly process, a new zero-loadbase measurement is taken as a value just prior to the load-inducingassembly stage by selecting or calculating a load measurement prior tothe load-inducing stage. This can be achieved by selecting a loadmeasurement corresponding to a fixed time or angle prior to thedetection of the commencement of the load-inducing stage, for example.Alternatively, for through-hole applications, the end of thethread-forming phase can be detected by a reduction in torque. It isagain to be understood that such methods are only illustrative, and thatthere are numerous other methods for determining the new zero-load basemeasurement prior to tightening, from load, time, torque and angle ofrotation measurements recorded during assembly operations with hand andpowered assembly tools.

The thermal effect of thread forming causes an apparent positive loadvalue at zero load just prior to tightening. An alternative to zeroingthe load (or time-of-flight measurement) is to add this load offset,measured prior to the load-inducing stage of the assembly process, tothe target load (or target time-of-flight). The result is the same sincethe increase in measured load is the same.

Yet another alternative is to experimentally determine an average valueof load error due to the thread forming and adjust the zero-loadmeasurement or target tightening parameter to compensate for this effectusing one of the above-described methods. This approach, however, doesnot compensate for variations with individual fasteners or jointcomponents and is therefore presently considered less desirable.

The result is that, for the first time, ultrasonic load measurementtechnology can be used with thread-forming fasteners. Errors in loadmeasurement resulting from the thermal effects of thread-forming can becompensated. This then results in accurate load measurement andtightening control of the thread-forming fasteners.

The above-described method of eliminating the effects of fastenerheating resulting from the thread-forming process can also be used withother fastener assembly processes that generate heat prior to theload-inducing tightening stage. Thread-locking bolts and nuts, forexample, are manufactured with a prevailing “locking” torque to preventthe fastener from loosening during service. Most often, the thread ofeither the bolt or nut has an irregular profile causing the threads toelastically deform slightly upon mating. Alternatively, the bolt or nuthas an insert or patch of a soft material to provide the prevailingtorque or resistance to loosening. The prevailing torque provided bythese thread-locking features produces heating of the fastener duringrundown in the same manner as the tapping torque does with athread-forming fastener. Consequently, the above-described method forcompensating for thermal-related errors in accordance with the presentinvention can be used with prevailing torque-locking fasteners toimprove the accuracy of ultrasonic load measurement during assembly.

It will be understood that various changes in the details, materials andarrangement of parts which have been herein described and illustrated inorder to explain the nature of this invention may be made by thoseskilled in the art within the principle and scope of the invention asexpressed in the following claims.

1. An apparatus comprising: a thread-forming fastener including a headfor engagement by a tool for applying a torque to the fastener, and abody portion extending from the head and including thread-formingportions; and an ultrasonic transducer coupled with the fastener, formaking ultrasonic load measurements in the fastener.
 2. The apparatus ofclaim 1 wherein the ultrasonic transducer is coupled with the head ofthe fastener.
 3. The apparatus of claim 1 wherein the ultrasonictransducer is permanently attached to the fastener.
 4. The apparatus ofclaim 3 wherein the ultrasonic transducer is comprised of apiezoelectric polymer film permanently attached to the head of thefastener.
 5. The apparatus of claim 3 wherein the ultrasonic transduceris comprised of an oriented piezoelectric thin film, vapor depositeddirectly on the head of the fastener.
 6. The apparatus of claim 3wherein the ultrasonic transducer is chemically grafted on the head ofthe fastener.
 7. The apparatus of claim 1 wherein the ultrasonictransducer is temporarily attached to the fastener.
 8. The apparatus ofclaim 1 wherein the ultrasonic transducer further includes aninformation storage medium applied to the ultrasonic transducer, whereinthe information storage medium includes markings corresponding to dataassociated with the fastener.
 9. The apparatus of claim 8 wherein theinformation storage medium is a bar code applied to the ultrasonictransducer.
 10. A method of making a load indicating, thread-formingfastener, comprising the steps of: providing a fastener having a firstend including a surface for receiving an ultrasonic transducer, formaking ultrasonic load measurements in the fastener, a shank extendingfrom the first end and including thread-forming portions for tapping ahole, and a second end, opposite the first end and including a surfacefor reflecting an ultrasonic wave back to the first end; and attachingan ultrasonic transducer to the first end of the fastener.
 11. Themethod of claim 10 which further includes the step of attaching theultrasonic transducer to a head associated with the first end of thethread forming fastener, for engagement by a tool for applying a torqueto the fastener.
 12. The method of claim 10 which further includes thestep of permanently attaching the ultrasonic transducer to the fastener.13. The method of claim 12 which further includes the step ofpermanently attaching an ultrasonic transducer comprised of apiezoelectric polymer film to the first end of the fastener.
 14. Themethod of claim 12 which further includes the step of vapor depositingan ultrasonic transducer comprised of an oriented piezoelectric thinfilm directly onto the first end of the fastener.
 15. The method ofclaim 12 which further includes the step of chemically grafting anultrasonic transducer onto the first end of the fastener.
 16. The methodof claim 10 which further includes the step of temporarily attaching theultrasonic transducer to the fastener.
 17. The method of claim 10 whichfurther includes the step of applying an information storage medium tothe ultrasonic transducer, wherein the information storage mediumincludes markings corresponding to data associated with the fastener.18. The method of claim 17 which further includes the step of applying abar code to the ultrasonic transducer.
 19. A method of measuring a loadin a fastener, comprising the steps of: providing a thread-formingfastener including a head for engagement by a tool for applying a torqueto the fastener, and a body portion extending from the head andincluding thread-forming portions; coupling an ultrasonic transducerwith the fastener, for making ultrasonic load measurements in thefastener; electrically connecting an apparatus to the ultrasonictransducer for supplying signals to the ultrasonic transducer and forreceiving signals from the ultrasonic transducer; monitoring the signalsreceived from the ultrasonic transducer, providing an accuratemeasurement indicative of the load in the fastener; and adjusting themeasurement indicative of the load to compensate for effects of heatingof the fastener resulting from forming a thread in a mating componentduring installation.
 20. The method of claim 19 which further includesthe step of imparting torque to the fastener and removing torque fromthe fastener in response to the measurement of the load in the fastener.21. The method of claim 19 which further includes the step ofdetermining a zero-load ultrasonic measurement, using the measurementindicative of the load in the fastener.
 22. The method of claim 21 whichfurther includes the step of measuring a torque in conjunction with themeasurement indicative of the load in the fastener to determine thezero-load ultrasonic measurement.
 23. The method of claim 21 whichfurther includes the step of measuring an angle of rotation of thefastener in conjunction with the measurement indicative of the load inthe fastener to determine the zero-load ultrasonic measurement.
 24. Themethod of claim 21 which further includes the step of measuring time inconjunction with the measurement indicative of the load in the fastenerto determine the zero-load ultrasonic measurement.
 25. The method ofclaim 21 which further includes the step of taking measurements fordetermining the zero-load ultrasonic measurement prior to aload-inducing stage of the installation.
 26. The method of claim 21which further includes the step of taking measurements for determiningthe zero-load ultrasonic measurement during a load-inducing stage of theinstallation.
 27. The method of claim 19 which further includes the stepof placing markings on the ultrasonic transducer corresponding to dataassociated with the fastener.
 28. The method of claim 27 which furtherincludes the step of marking a bar code on the ultrasonic transducer.29. A method of measuring a load in a fastener, comprising the steps of:providing a thread-locking fastener assembly including a bolt having ahead for engagement by a tool for applying a torque to the fastenerassembly and a body portion extending from the head, and a nut forcooperating with the body portion of the bolt, wherein portions of thefastener assembly include resistance-inducing threads; coupling anultrasonic transducer with the bolt, for making ultrasonic loadmeasurements in the fastener assembly; electrically connecting anapparatus to the ultrasonic transducer for supplying signals to theultrasonic transducer and for receiving signals from the ultrasonictransducer; monitoring the signals received from the ultrasonictransducer, providing an accurate measurement indicative of the load inthe fastener assembly; and adjusting the measurement indicative of theload to compensate for effects of heating of the fastener assemblyresulting from prevailing torque associated with the thread-lockingfastener assembly.
 30. The method of claim 29 which further includes thestep of imparting torque to the bolt and removing torque from the boltin response to the measurement of the load in the fastener assembly. 31.The method of claim 29 which further includes the step of determining azero-load ultrasonic measurement, using the measurement indicative ofthe load in the fastener assembly.
 32. The method of claim 31 whichfurther includes the step of measuring a torque in conjunction with themeasurement indicative of the load in the fastener assembly to determinethe zero-load ultrasonic measurement.
 33. The method of claim 31 whichfurther includes the step of measuring an angle of rotation of thefastener in conjunction with the measurement indicative of the load inthe fastener assembly to determine the zero-load ultrasonic measurement.34. The method of claim 31 which further includes the step of measuringtime in conjunction with the measurement indicative of the load in thefastener assembly to determine the zero-load ultrasonic measurement. 35.The method of claim 31 which further includes the step of takingmeasurements for determining the zero-load ultrasonic measurement priorto inducing the load in the fastener assembly.
 36. The method of claim31 which further includes the step of taking measurements fordetermining the zero-load ultrasonic measurement while inducing the loadin the fastener assembly.
 37. The method of claim 29 which furtherincludes the step of placing markings on the ultrasonic transducercorresponding to data associated with the fastener assembly.
 38. Themethod of claim 37 which further includes the step of marking a bar codeon the ultrasonic transducer.